1. Field of the Invention
The present invention relates to a method for controlling switching in a bidirectional line-switched ring (BLSR), more particularly relates to a method for controlling switching in a bidirectional line-switched ring which can be applied to a bidirectional line-switched ring network having a four-fiber configuration of the North American SONET standard.
The method for controlling switching in a bidirectional line-switched ring (BLSR) of SONET is realized by SONET Standard GR-1230-CORE Issue 4. In this method, there are two modes for switching in a ring network: ring switch and span switch. To restore a span in which a failure occurs, use is made of a protection channel in the ring network in both the cases of ring switch and span switch.
In this case, the switching is executed between two adjacent nodes connected to the two sides of the span to be switched.
In the BLSR switching control method, information concerning the switching is transferred to nodes of the ring network by utilizing the K1 and K2 bytes in a section overhead (SOH). The method for utilizing these K1 and K2 bytes (K bytes) is defined in SONET Standard GR-1230-CORE Issue 4.
2. Description of the Related Art
The present invention deals with the problems occurring due to the fact that ring switch can co-exist with another switch request on an identical span. As one means for dealing with these problems, there is for example the technique disclosed in Japanese Unexamined Patent Publication (Kokai) No. 9-18419. Japanese Unexamined Patent Publication (Kokai) No. 9-18419, however, alludes to the problem occurring when releasing ring switch when ring switch and an LP-S co-exist and differs from the present invention.
Namely, the present invention deals with the problems arising where ring switch and an LP-S cannot co-exist, but ring switching and SF-P can.
Specifically, the present invention deals with problems arising on the same side (span side) of each node where:
(i) LP-S and ring switch by a command (FS-R, MS-R, and EXER-R) cannot be executed, and
(ii) LP-S and ring switch due to channel failure (SF-R, SD-R) cannot be executed, but
(iii) SF-P and ring switch by a command (FS-R, MS-R, and EXER-R) are executed, or
(iv) SF-P and ring switch due to channel failure (SF-R, SD-R) are executed.
When dealing with this situation, three problems arise. These problems will be explained in detail later by referring to the drawings, but in brief are as follows:
The first problem is such a problem that the first 4 bits of the K1 byte indicating a switch request are “1111” in both the LP-S and SF-P, so these cannot be differentiated. In some cases, even if there is a request for ring switch on the identical span, the ring switch cannot be executed.
The second problem is that during the execution of LP-S, the user receiving special communication service (protection channel access: PCA) can no longer use a protection channel in use for the PCA.
The third problem is that if there is a change in a switch request after an LP-S or SF-P switch request, the change cannot be quickly followed up on.